Light emitting diodes (“LEDs”) are, in general, miniature semiconductor devices that employ a form of electroluminescence resulting from the electronic excitation of a semiconductor material to produce visible light. Initially, the use of these devices was limited mainly to display functions on electronic appliances and the colors emitted were red and green. As the technology has improved, LEDs have found a wide range of new uses and applications.
In the manufacturing process, the LEDs may be mounted on a substrate that may include an integrated circuit (“IC” or “chip”) or a printed circuit board (“PCB”), and may also have terminals or leads attached. It is appreciated by those skilled in the art that each of the LEDs or substrate may be described as a “die,” which refers to an individual monolithic device and therefore may be interchangeably used with the terms “IC” and “chip.” The LEDs may be mounted using conductive paste because the substrate may form one of the two diode connections. The LEDs may also be wire bonded to an interconnection pattern. Depending on how the LEDs are internally connected in the package, LEDs may be referred to as a Common Anode or Common Cathode type, with the former being predominant. There are also versions where all LED connections have been wire bonded out.
An LED may be positioned in a concave base cavity adapted to provide an initial focus for the light output from the LED. The LED may be provided with anode and cathode bonding wires that place the LED in communication with an electrical circuit for supplying a bias voltage to the LED. The LED may be encapsulated in a material intended to protect the LED from external contaminants and from being physically damaged or dislodged, and which may form part of a lens system for further focusing the light output of the LED. A substrate on which the LED rests may include a metallized portion underneath the LED that may serve to dissipate heat from the LED.
Along with light output, LED devices also generate heat. Despite typical design features of LED devices including those summarized above, LED devices are commonly prone to damage caused by the buildup of heat generated from within the devices, as well as heat from sunlight in the case of outside lighting applications. Although metallized LED substrates are useful design elements that may be incorporated in LED devices and can serve to dissipate heat, these elements are often inadequate to maintain reasonably moderate temperatures in the devices that will ensure adequate maximum drive current without compromising the reliability of the package. For example, the semiconductor junction temperature, which increases as more current is applied, may be high enough to cause the encapsulant placed in the LED to expand and contract, thus causing displacement of the wire bonding and resulting in premature wear-out and even failure of the LED. Excessive heat buildup can also cause deterioration of the materials in the LED devices, such as the encapsulants for the LED.
Epoxy and silicone polymers, commonly used in LED encapsulant formulations, generally are poor heat conductors and are not sufficiently resistant to the high temperatures that often are generated inside LED devices during operation. These polymers can develop substantially reduced light transmissivity as they undergo heat degradation caused by such high temperatures. This reduced light transmissivity can increase absorbance by the LED devices themselves of light at wavelengths that are intended to be included in light output from the devices. This light absorbance can be pronounced at near-ultra-violet wavelengths, and can cause commensurate declines in the light output quality and intensity from an LED device.
An additional problem that is exacerbated by excessive heat in the LED device is that of Coefficient of Thermal Expansion (“CTE”) mismatch between the encapsulant material, the die, and the bonding wires. In general, thermal stresses may be caused by the differential expansion of the die and its substrate, the encapsulant material and the bonding wires, and the encapsulant material and the die. One approach is to match the CTE of the encapsulant material and the bonding wires to reduce thermal stresses and prevent displacement of the wire bonding. Also the encapsulant material may be chosen for either a rigid or a compliant package, which is also a factor in reducing thermal stresses.
Consequently, there is a continuing need to provide new LED device structures having improved capability to dissipate heat in order to protect against the degradation of certain elements of the LED device.